The present invention relates to an antishrinkage admixture for concrete or mortar.
To compensate for the shrinkage of a concrete or mortar, it is customary to use an expansive process, namely a micronized aluminum powder, a calcium carbide, a quicklime, hydrogen peroxide, a calcium aluminosulfate or an iron powder is mixed with the mortar or concrete composition in the dry state.
The swelling obtained is about 300 to 800 microns per meter at 28 days. Since the shrinkage of mortar or concrete in the cured state is of the same order, for example 600 microns per meter for a 28-day standardized mortar, an antishrinkage effect is normally obtained.
In theory, the shrinkage is therefore well compensated for but, in reality, it is very difficult to control this expansion and the moment when it occurs.
Likewise, to limit the shrinkage it is possible to use various admixtures of the fatty acid type, polyalkylene glycol ethers, etc., which do not produce swelling but make it possible to limit the shrinkage by acting on the surface tension of the water in the pores of the hydrated cement. This type of admixture, although reducing the short-term (28 day) shrinkage, produces in reality very limited results in the longer term ( greater than 90 days).
According to document JP-A-01-172 252, a hardening admixture for concrete or mortar is described, which comprises:
a gypsum, namely a dehydrated calcium sulfate, or anhydrite, namely an anhydrous calcium sulfate;
triethanolamine or diethylene glycol; and
an hydroxylated carboxylic acid; or a salt of lignin and sulfonic acid.
In such a hardening admixture, the gypsum or anhydrite makes it possible to regularize the setting time of the cement into which said hardening admixture is incorporated.
As regards triethanolamine or diethylene glycol, their surfactant properties make it possible in some cases to improve the mechanical strength of certain concretes or mortars.
The Applicant has discovered novel antishrinkage admixtures having the ability to create specific microstructures in the cured concrete or mortar, with a greater reproducibility and reliability, with the various types of cement encountered in the market.
Surprisingly, it has been found that a calcium sulfate, and more particularly an anhydrous calcium sulfate, combined with at least one glycol ether makes it possible to obtain such a microstructure and is suitable as an antishrinkage admixture.